Wednesday, July 7, 2010

5.4 Earthquake in Southern California: Not an aftershock this time

A magnitude 5.4 quake has struck in southern California, but in this instance, I don't think this is an aftershock to the Easter Sunday El Mayor-Cucapah quake earlier this year. The quake took place on a different fault system, probably the Coyote Creek or San Jacinto fault. The constant aftershock activity from the El Mayor quake may very well have had a role in destabilizing the fault that moved today. As always, moderate quakes like this serve as a reminder that California is earthquake country, and of all natural disasters, these are the most unexpected. We can't predict them, so we have to be ready for them, and understand the faults and their history the best way we can. I am repeating a description of the San Jacinto fault that I wrote in February after an earthquake swarm in the Redlands area along the same fault system....

"A lot of people, when they hear of quakes in California, think San Andreas Fault. Many California residents who have lived here all their lives cannot think of the name of another fault in the state, but our landscape is literally crisscrossed by active faults (look at the map above; most of the brown lines are active faults). Most earthquakes in CA happen on faults other than the San Andreas, and this week's swarm is no exception. It appears to be taking place on one what is arguably the most active fault in the state, the San Jacinto fault.

The San Jacinto fault is certainly part of the San Andreas system. It splits off from the San Andreas at the east end of the San Gabriel Mountains, and runs roughly parallel to the San Andreas for 140 miles south into the Imperial Valley. It has the same type of motion, right lateral (features on the opposite side of the fault have been shifted to the observer's right). Since it began moving a few million years ago, something like 15 miles of lateral motion has taken place.

The San Andreas fault is justly famous for several devastating earthquakes, including the 1906 San Francisco event that killed 3,000 people, and the 1989 Loma Prieta quake (the World Series quake). It also produced a magnitude 8 event in southern California in 1857. But most of the time the fault is quiet (in a menacing way; it is storing up stress). Compare this with the history of the San Jacinto system (courtesy of Wikipedia):

1918 San Jacinto Earthquake - Magnitude 6.9 earthquake strikes the same area that was damaged by an earthquake 19 years earlier, with an epicenter roughly 10 mi NW of the previous earthquake.

1923 North San Jacinto Fault Earthquake - Magnitude 6.3 earthquake damaged the San Bernardino and Redlands area. Last time the fault, which runs under the I-215/I-10 interchange, ruptured in this area.

1937 Terwilliger Valley Earthquake - Magnitude 6.0

1942 Fish Creek Mountains Earthquake - Magnitude 6.3

1954 Arroyo Salada Earthquake - Magnitude 6.2

1968 Borrego Mountain Earthquake - Magnitude 6.5

1987 Superstition Hills Earthquake - Magnitude 6.6 (Note: some consider it to have occurred on a fault completely unrelated to the San Jacinto Fault Zone)

These quakes are considerably smaller than the 1906 event, by a factor of 32 or more (it takes the energy of 32 magnitude six quakes to equal the energy of a single magnitude 7 quake; a magnitude 8 quake is 32 times more powerful than a magnitude 7 quake and more than a thousand times more powerful than a six). But they clearly happen more often . The message is clear: to live in California, we must be prepared not just for the BIG, HUGE ONE, but also lots of lesser BIG ONES."

The picture above is a linear valley along the San Jacinto fault near the mountain town of Idylwild in the San Jacinto Mountains. I was there four days ago. As usual I missed the quake.

3 comments:

Mark Allen
said...

Thanks for all the information - best news in California! My good friend who lives in Palm Springs said he felt it really shake, knocked a few things over, but no "real" damage. Then here in Downey at the new Kaiser Hospital my wife said it was "really shaking" - but only a few miles to the south in La Mirada where we live I felt nothing - only my 87 year old mother who was sitting in her room "thought" she felt a rolling motion for a few seconds. I figured the new hospital is probably one of the most recent major structures locally, and moved so much as that is what it is suppose to do? What do you think? Mark.

The upper stories of most buildings shake more than the lower floors, so people are more likely to feel it. Also, people who are laying or sitting also are more likely to feel a quake. If you are standing, your inner ear automatically adjusts your balance subconciously, so you don't notice the ground moving. Thanks for commenting!

About Me

I am a teacher of geology at Modesto Junior College and former president of the National Association of Geoscience Teachers, Far Western Section. I have led field trips all over the western United States, and a few excursions overseas, but my homebase is the Sierra Nevada, the Great Valley, and the Coast Ranges of California.